In this paper the operation of capacitive soil moisture sensors are modeled using an electrical circuit analogue. This model aims to predict the response of capacitive sensors for a variety of soil types, moistures, soil conductivity and sensor operating frequencies. The model is extensively validated under a variety of conditions for a variety of sensor circuits and measurement techniques. The deposition of a conducting film composed of clay-like soil material over the sensing surface of a soil moisture sensor is shown to be the cause of hysteresis when the sensor is operated at low frequencies (10KHz). As the frequency is increased (10MHz) the effect of the conducting film becomes insignificant. Surface chemistry analysis techniques were used to identify the soil deposits on the conducting film. This research is motivated by the design of a small disposable sensor printed on a flexible plastic substrate measuring soil moisture as a function of the number of point contacts terminating on the insulated sensor electrode. In controlled conditions the sensor exhibits a linear response across most of its range to water content changes, but in some soils the reading becomes "stuck" on a high reading and does not return to a lower reading until the soil has dried considerably.
In a sensor employing changes in capacitance the introduction of two additional reference electrodes can assist in the minimization and correction of errors introduced in the manufacturing process and from changes in environmental conditions. The two extra electrodes that reflect the maximum and minimum values of the measuring electrode are constructed during the same lithographic process and in close proximity to the active electrode. In preliminary trials of 79 sensors the uncorrected error in reading was nearly 20% of full scale and dropped to 4% of full scale after applying the correction. The technique with support electronics printed on flexible substrates allow the sensor to be small, integrated and “smart”.